Group Index (AASHTO/IS) calculation from gradation and Atterberg limits Given: fraction passing 0.075 mm sieve F = 60%, liquid limit LL = 65%, and plastic limit PL = 40% (so PI = 25%). Compute the soil’s Group Index (GI).

Introduction / Context:The Group Index (GI) is an empirical rating used primarily in the AASHTO system (also referenced in Indian practice) to evaluate the quality of fine-grained subgrade soils. It combines gradation (percentage passing 0.075 mm) and plasticity characteristics (LL and PI).

Given Data / Assumptions:

• F = 60% passing 0.075 mm.
• LL = 65%.
• PL = 40% ⇒ PI = LL − PL = 25%.
• Use the standard GI expression with capping of terms where applicable.

Concept / Approach:

The standard Group Index formula is: GI = (F − 35) * [0.2 + 0.005 * (LL − 40)] + 0.01 * (F − 15) * (PI − 10) with bracketed terms truncated to the range 0 to 40 or 0 to 20 as required by practice (i.e., do not allow negative values; also cap excess where appropriate).

Step-by-Step Solution:

Verification / Alternative check:

GI is rounded to the nearest whole number: 14. This is not among 5, 20, or 40.

Why Other Options Are Wrong:

5, 20, and 40 do not match the computed GI; the correct value is approximately 14, thus “none of these” fits the provided choices.

Common Pitfalls:

Forgetting to cap the intermediate terms; arithmetic slips with decimal multipliers; mixing LL and PI locations in the formula.

Final Answer:

None of these (the GI ≈ 14)